The present invention relates to a power supply device adapted for various electric or electronic instruments and more particularly to a switching mode power supply controller having two-output-levels minimize the power consumption without the use of a relay circuit in a stand-by site by reducing the level of one of the output voltages of the power supply controller.
In general, the conventional instruments employing the switching mode power supply control the switching frequency or the pulse width to provide constant output voltages, for example, +124, +24 and +16 V, to the corresponding output terminals, respectively. In the stand-by state for which a master switch of the switching mode power supply is in ON-state, the switching mode power supply provides the power only to the necessary parts, for example, a microcomputer and the like and the output voltages for the other parts are cut off by a relay circuit.
On the other hand, if the system does not include the relay circuit, the output voltages are controlled by an ON/OFF switching operation of transistors in accordance with an input supply voltage however, there remain many difficulties when a power supply device having many output terminals to be cut off for the power in the stand-by state. In addition, if none of the output voltages are cut off except the input supply voltage, unnecessary power is consumed in the stand-by state.